Piezoelectric bulk acoustic wave (BAW) resonators are widely employed in high performance radio-frequency (RF) filters and duplexers in wireless handsets. Compared to conventional RF ceramic filters and surface acoustic wave (SAW) filters, BAW filters have advantages of small form factor, low frequency drift with temperature, robust power handling, high frequency operation, etc. Combined with integrated circuits (IC), BAW resonators have also been used to build low phase noise oscillators and voltage controlled oscillators for frequency control applications. BAW resonators are usually manufactured on silicon wafers using IC fabrication processing techniques, which have a small footprint and low profile, very cost effective.
As shown in FIG. 11, a conventional BAW resonator 10 includes a piezoelectric layer 14, such as aluminum nitride, sandwiched between two metal electrodes 14a and 14b. When an RF electric signal is applied across the two electrodes 14a and 14b, the resonator body is mechanically extended and contracted due to the piezoelectric effect and an acoustic wave is excited in the structure 10. The acoustic wave propagates parallel to the applied electric field and is reflected at the interfaces of the electrodes 14a and 14b and air to form the resonance.
One example of a BAW resonator is a thin film bulk acoustic resonator (FBAR). The resonator includes a piezoelectric layer between bottom and top metal electrodes. The overlapping area of the two electrodes is the active area and acoustic energy is trapped there. Both sides of an FBAR are interfaced with air or vacuum, for example, an air cavity 13 formed on or in the substrate 11, to confine the acoustic wave. In practical implementation, the active area of the structure is suspended over a substrate. The bottom air interface is formed either using a sacrificial layer (which is then removed), or realized by etching part of the substrate away. The substrate is typically silicon, although other substrate materials can be used.
There is a second type of BAW resonator known as a solidly mounted resonator (SMR). In the SMR structure, the bottom electrode is mounted above an acoustic mirror stack comprising multiple reflective layers of low and high acoustic impedance materials, for example, low density silicon oxide and high density tungsten. The mirror stack replaces the air interface below the lower electrode in the FBAR structure, and provides isolation between the resonator and the silicon substrate, preventing acoustic losses into the substrate.
Resonant frequency of a BAW resonator is mainly determined by the thicknesses of all layers in the path of the acoustic wave propagation. The deposited films are not perfectly uniform, resulting in the device resonant frequency to have a distribution over the complete wafer. A frequency trimming is necessary to reduce the frequency distribution from several tens MHz down to several MHz to achieve a decent yield. In a BAW resonator, a trimming sensitivity is defined as a shift or change of resonant frequency of the BAW resonator with respect to change of top layer thickness and can be expressed in kHz per Angstrom. For example, a trimming sensitivity of 10 kHz/Angstrom indicates that a thickness of 1 Angstrom change of the top layer can cause a 10 kHz change in the resonant frequency of the resonator. The trimming sensitivity is proportional to the resonant frequency and typically in the range of 25-100 kHz/Å for 2 GHz resonators. When a BAW resonator frequency increases up to 5 GHz and the film layers in the resonator stack are made thinner, the trimming sensitivity could be as high as 250 kHz/Å. Trimming a wafer with such a high trimming sensitivity to a tight frequency distribution becomes extremely challenging.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.